• Loopt: As a college student at Stanford, Sam Altman wanted to know where his classmates were hanging out after class. So he created Loopt, a mobile-based GPS sharing system that lets users visualize and map where their friends are located. Today, more than 5 million users rely on the app to find out where to meet up with friends through detailed interactive maps on their cell phones. Loopt is continually innovating its product as well. In March, it launched an opt-in Reward Alerts program that sends out deals in real time depending on the user’s location. The impulse buyer seems to be the main target considering the deals are issued on a first-come, first-served basis with a cap on how many discounts one can get.

• Foursquare: What started as a graduate thesis became a reality for Foursquare founders Dennis Crowley and Naveen Selvadurai. The two-year-old location-based social networking Web site is available to anyone with GPS-enabled devices. Users can check in at venues, score points, and earn badges based on number of visits to a particular place. Businesses also can run “specials” that are sent to users based on their location.

• Google Latitude: Similar to the other tools, Google Latitude is a popular tool that permits consumers to meet up with people in their network using mobile devices or PCs. The application is integrated with Google Maps, giving users the ability to place their friends directly on a map.

• Facebook Places: When Facebook Beacon was shut down after a class action lawsuit in 2009, Mark Zuckerberg went back to the drawing board. Facebook Beacon was part of the social network’s advertising system that sent data from external Web sites directly to Facebook to create personalized advertisements for users. Today, Facebook Places gives users the option to check in and update friends about where they are and what they are doing. In addition, users receive personalized discounts depending on the location of their check-ins. The service operates on an opt-in basis and is available through PC and mobile applications.

• Gowalla: This Austin-based social network is primarily a mobile application designed for users to check in at locations on their smartphone. As an incentive, users receive rewards for their check-ins that can be swapped or dropped at check-in spots. Similar to Foursquare, as users rack up check-ins at a particular spot, they may earn elevated status above normal users if they drop an item.

• Groupon: From online coupons exclusively for the Chicagoland area to a global deal finder, Groupon has established itself as a major player in the geolocation app world in only a couple of years. Today, users can download the app on their smartphone to search for nearby deals.